As is well known in the art, wheel lock-up and vehicle skidding can be caused by severe slippage between the wheel and the driving surface. In many cases, lock-up increases required stopping distance and reduces directional control of the vehicle.
Such problems have been generally addressed with the advent of antilock brake systems (ABS). A typical ABS is designed to identify an excessive wheel slip condition by comparing the measured velocity of a wheel to a reference speed, which is an estimate of the true vehicle speed based on current and previous values of individual wheel velocities. If the velocity of a particular wheel is significantly less than the reference speed, then that wheel is determined to be experiencing excessive slip. In response, hydraulic pressure actuating a corresponding brake is modulated to reduce brake torque, thereby reducing braking force between the wheel and driving surface which, in turn, reduces wheel slip.
In practice, ABS first isolates existing brake fluid in an individual wheel brake from increasing brake fluid pressure in the master cylinder in order to hold pressure in the brake constant. ABS then dumps fluid from the brake to reduce pressure therewithin. Thereafter, ABS typically holds pressure in the brake constant for a selected amount of time.
After a period of constant pressure following pressure reduction, pressure is then increased until excessive wheel slip occurs again. The resulting cycle of decreasing, maintaining, and then increasing pressure is repeated until excessive slip no longer occurs. The specifics of this brake pressure cycle depend on the particular algorithm employed within the ABS logic control unit, along with vehicle characteristics and the driving surface conditions encountered at the time of braking.
One parameter which represents driving surface conditions is the coefficient of friction, commonly denoted by mu (.mu.). Two classes of surfaces can be defined qualitatively in terms of .mu.. A high .mu. surface is one wherein relatively good braking ability is possible, such as dry asphalt. A low .mu. surface is one wherein relatively poor braking ability is possible, such as a snow or ice-covered road, or wet asphalt.
Certain types of driving surfaces, such as gravel, packed snow, and certain types of ice, are known in the art as "deformable" surfaces. On non-deformable surfaces, ABS forces wheel departures on a regular basis where wheel slip will exceed the optimum (peak) value for short periods of time. In contrast, on deformable surfaces, surface irregularities can cause apparent departures well before the peak slip is reached. Due to irregular wheel decelerations, pressure induced departures can also cause premature dumping and quick wheel speed recovery. In both cases, however, the departure depths are usually small.
Although ABS helps to decreased stopping distance on many surfaces, it is also known in the art that locked wheels actually improve stopping distance on deformable surfaces. Even so, locked wheels are not necessarily desirable on deformable surfaces because reduced directional control of the vehicle persists.
For this reason, it would be desirable for ABS to modify brake pressure control when the vehicle is braked on a deformable surface wherein locked wheels improve stopping distance.